Method and apparatus for multiple frame transmission for supporting MU-MIMO

ABSTRACT

A method of multi user multiple input multiple output (MIMO) communication in a wireless local area network (WLAN) system; and a transmitting device therefore are discussed. The method according to an embodiment includes generating an Aggregate-Medium Access Control Protocol Data Unit (A-MPDU) including an A-MPDU subframe, the A-MPDU subframe including a Medium Access Control Protocol Data Unit (MPDU) delimiter field; and selecting a type of a guard interval. When the A-MPDU is generated for a plurality of receiving devices, the transmitting device selects a short guard interval only if all of the plurality of receiving devices support the short guard interval. The MPDU delimiter field includes a null field and an MPDU length field, the null field indicates a corresponding A-MPDU subframe is used to pad the A-MPDU, and the MPDU length field indicates a length of an MPDU. The null field is a 1-bit information field.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.13/392,216 filed on Feb. 24, 2012, which is the National Phase ofPCT/KR2010/004502 filed on Jul. 12, 2010, which claims the benefit under35 U.S.C. 119(e) of U.S. Provisional Application No. 61/236,887 filed onAug. 26, 2009 and 61/245,656 filed Sep. 24, 2009. The contents of all ofthese applications are hereby incorporated by reference as fully setforth herein in their entirety.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to wireless communications, and moreparticularly, to a method of transmitting multiple frames in a wirelesslocal area network (WLAN) system supporting multi-user multiple inputmultiple output (MU-MIMO) and a wireless apparatus supporting themethod.

Discussion of the Related Art

With the advancement of information communication technologies, variouswireless communication technologies have recently been developed. Amongthe wireless communication technologies, a wireless local area network(WLAN) is a technology whereby Internet access is possible in a wirelessfashion in homes or businesses or in a region providing a specificservice by using a portable terminal such as a personal digitalassistant (PDA), a laptop computer, a portable multimedia player (PMP),etc.

Ever since the institute of electrical and electronics engineers (IEEE)802, i.e., a standardization organization for WLAN technologies, wasestablished in February 1980, many standardization works have beenconducted. In the initial WLAN technology, a frequency of 2.4 GHz wasused according to the IEEE 802.11 to support a data rate of 1 to 2 Mbpsby using frequency hopping, spread spectrum, infrared communication,etc. Recently, the WLAN technology can support a data rate of up to 54Mbps by using orthogonal frequency division multiplex (OFDM). Inaddition, the IEEE 802.11 is developing or commercializing standards ofvarious technologies such as quality of service (QoS) improvement,access point protocol compatibility, security enhancement, radioresource measurement, wireless access in vehicular environments, fastroaming, mesh networks, inter-working with external networks, wirelessnetwork management, etc.

The IEEE 802.11n is a technical standard relatively recently introducedto overcome a limited data rate which has been considered as a drawbackin the WLAN. The IEEE 802.11n is devised to increase network speed andreliability and to extend an operational distance of a wireless network.More specifically, the IEEE 802.11n supports a high throughput (HT),i.e., a data processing rate of up to above 540 Mbps, and is based on amultiple input and multiple output (MIMO) technique which uses multipleantennas in both a transmitter and a receiver to minimize a transmissionerror and to optimize a data rate. In addition, this standard may use acoding scheme which transmits several duplicate copies to increase datareliability and also may use the OFDM to support a higher data rate.

With the widespread use of the WLAN and the diversification ofapplications using the WLAN, there is a recent demand for a new WLANsystem to support a higher throughput than a data processing ratesupported by the IEEE 802.11n. However, an IEEE 802.11n medium accesscontrol (MAC)/physical layer (PHY) protocol is not effective to providea throughput of above 1 Gbps. This is because the IEEE 802.11n MAC/PHYprotocol is designed for an operation of a station (STA), that is, anSTA having one network interface card (NIC), and thus when a framethroughput is increased while conforming to the conventional IEEE802.11n MAC/PHY protocol, a resultant additional overhead is alsoincreased. Consequently, there is a limitation in increasing athroughput of a wireless communication network while conforming to theconventional IEEE 802.11n MAC/PHY protocol, that is, a single STAarchitecture.

Therefore, to achieve a data processing rate of above 1 Gbps in thewireless communication system, a new system different from theconventional IEEE 802.11n MAC/PHY protocol (i.e., the single STAarchitecture) is required. A very high throughput (VHT) WLAN system is anext version of the IEEE 802.11n WLAN system, and is one of IEEE 802.11WLAN systems which have recently been proposed to support a dataprocessing rate of above 1 Gbps in a MAC service access point (SAP).

The VHT WLAN system allows simultaneous channel access of a plurality ofVHT STAs for the effective use of a radio channel. For this, amulti-user multiple input multiple output (MU-MIMO)-based transmissionusing multiple antennas is supported. The VHT AP can perform spatialdivision multiple access (SDMA) transmission for transmitting spatiallymultiplexed data to the plurality of VHT STAs.

However, when frames are simultaneously transmitted to a plurality ofSTAs in a WLAN system supporting MU-MIMO, an amount of data to betransmitted to each STA may differ, and thus synchronization may not bemaintained between STAs. As a result, efficiency in the use of radioresources decreases and complexity of the STA increases, which leads toincrease in implementation costs. Such a problem may become moreapparent when multiple frames are transmitted for each of the pluralityof STAs. Accordingly, there is a need to consider a frame transmissionmethod capable of solving this problem.

SUMMARY OF THE INVENTION

In an aspect of the present invention, a method of transmitting multipleframes in a wireless local area network (WLAN) system supporting multiuser-multiple input multiple output (MU-MIMO) includes transmitting afirst frame and a second frame consecutively to a first station (STA),and transmitting a third frame and a fourth frame consecutively to asecond STA, wherein a transmission start time of the first frame and atransmission start time of the third frame are aligned to each other,and wherein a transmission start time of the second frame and atransmission start time of the fourth frame are aligned to each other.

A length of the first frame and a length of the third frame may beadjusted to the same length by padding null data to a shorter framebetween the first frame and the third frame by a difference between thelength of the first frame and the length of the third frame.

The first frame and the third frame may have an aggregate MAC protocoldata unit (A-MPDU) format.

Each of A-MPDU subframes constituting the first frame may include a nullbit for indicating whether a follow-up A-MPDU subframe is null data.

If the null bit indicates that an A-MPDU subframe following the A-MPDUsubframe comprising the null bit is null data, the first STA may discardthe A-MPDU subframe following the A-MPDU subframe comprising the nullbit.

In another aspect of the present invention, a method of transmittingmultiple frames, performed by an access point (AP), in a WLAN systemsupporting MU-MIMO includes transmitting a first frame and a secondframe consecutively to a first STA, and transmitting a third frame and afourth frame consecutively to a second STA, wherein a transmission starttime of the first frame and a transmission start time of the third frameare aligned to each other, and wherein an interval between the firstframe and the second frame and an interval between the third frame andthe fourth frame are set to a multiple of an orthogonal frequencydivision multiplex (OFDM) symbol duration.

The OFDM symbol duration may be 4 μs.

The first STA and the second STA may receive inter-frame space (IFS)configuration information from the AP, and in the IFS configurationinformation, the interval between the first frame and the second frameand the interval between the third frame and the fourth frame may be setto a multiple of the OFDM symbol duration.

The IFS configuration information may be transmitted to the first STAand the second STA by using a beacon frame.

In still another aspect of the present invention, an AP for transmittingmultiple frames includes a transceiver for transmitting the multipleframes, and a processor operationally coupled to the transceiver,wherein the processor transmits a first frame and a second frameconsecutively to a first STA, transmits a third frame and a fourth frameconsecutively to a second STA, aligns a transmission start time of thefirst frame and a transmission start time of the third frame to eachother, and aligns a transmission start time of the second frame and atransmission start time of the fourth frame to each other.

According to the present invention, an overhead is reduced in multipleframe transmission. Therefore, radio resources are more effectivelyused, and complexity of a wireless apparatus is decreased, thereby beingable to save implementation costs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of frame transmission using a multiuser-multiple input multiple output (MU-MIMO) scheme.

FIG. 2 shows an example of multiple frame transmission.

FIG. 3 shows a problem in which synchronization is not maintainedbetween frames to be transmitted to respective STAs in SDMAtransmission.

FIG. 4 shows an example of a slotted RIFS proposed in the presentinvention.

FIG. 5 shows an example of a synchronized multiple frame transmissionmethod proposed in the present invention.

FIG. 6 shows an example of an A-MPDU subframe format used in null datapadding according to an embodiment of the present invention.

FIG. 7 is a block diagram showing an example of a wireless apparatus forimplementing an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, exemplary embodiments of the present invention will bedescribed with reference to the accompanying drawings.

A wireless local area network (WLAN) system according to an embodimentof the present invention includes at least one basic service set (BSS).The BSS is a set of stations (STAs) successfully synchronized tocommunicate with one another. The BSS can be classified into anindependent BSS (IBSS) and an infrastructure BSS.

The BSS includes at least one STA and an access point (AP). The AP is afunctional medium for providing a connection to STAs in the BSS throughrespective wireless media. The AP can also be referred to as otherterminologies such as a centralized controller, a base station (BS), ascheduler, etc.

The STA is any functional medium including a medium access control (MAC)and wireless-medium physical layer (PHY) interface satisfying the IEEE802.11 standard. The STA may be an AP or a non-AP STA. Hereinafter, theSTA refers to the non-AP STA unless specified otherwise. The STA mayalso be referred to as other terminologies such as a user equipment(UE), a mobile station (MS), a mobile terminal (MT), a handheld device,an interface card, etc.

The STA can be classified into a VHT-STA, an HT-STA, and a legacy(L)-STA. The HT-STA is an STA supporting IEEE 802.11n. The L-STA is anSTA supporting a previous version of IEEE 802.11n, for example, IEEE802.11a/b/g. The L-STA is also referred to as a non-HT STA.

Hereinafter, transmission from the AP to the STA will be referred to asdownlink transmission, and transmission from the STA to the AP will bereferred to as uplink transmission. Further, transmission of spatialdivision multiple access (SDMA) data spatially multiplexed with respectto a plurality of STAs will be referred to as SDMA transmission.Although a downlink transmission scenario will be described as anexample hereinafter for convenience of explanation, a multiple frametransmission method proposed in the present invention can also equallyapply to an uplink transmission scenario.

FIG. 1 shows an example of frame transmission using a multiuser-multiple input multiple output (MU-MIMO) scheme.

In the example of FIG. 1, an AP 100 transmits SDMA data 104 spatiallymultiplexed with respect to an STA_1 110, an STA_2 120, and an STA_3 130by using the MU-MIMO scheme.

A plurality of physical layer convergence procedure (PLCP) protocol dataunits (PPDUs) may be used as a data frame to be transmitted for each ofthe STA_1 110, the STA_2 120, and the STA_3 130. When the plurality ofPPDUs are consecutively transmitted by the AP with an interval of ashort inter-frame space (SIFS) or a reduced inter-frame spacing (RIFS),it will be hereinafter referred to as multiple frame transmission.

To perform channel estimation with respect to a target STA for whichSDMA transmission is to be performed, the AP 100 transmits a trainingrequest (TRQ) frame 102 before SDMA transmission. The TRQ frame 102 mayinclude information indicating the target STA of SDMA transmission andinformation indicating a transmission duration. Upon receiving the TRQframe 102, an STA determines whether the STA itself is the target STA ofSDMA transmission by using the information which indicates the targetSTA of SDMA transmission and which is included in the TRQ frame 102. Ifit is not the target STA, the STA may set a network allocation vector(NAV) on the basis of information indicating the transmission durationso as to defer channel access during the transmission duration.

If it is determined that the STA is the target STA, the STA transmits tothe AP a sounding PPDU used for channel estimation with respect to aspecific STA. In the example of FIG. 1, the STA_1 110, the STA_2 120,and the STA_3 130 are target STAs, and transmit to the AP 100 a soundingPPDU 112, a sounding PPDU 122, and a sounding PPDU 132, respectively.

Upon receiving the sounding PPDU 112, the sounding PPDU 122, and thesounding PPDU 132, the AP 100 performs channel estimation by using thereceived sounding PPDUs. Thereafter, on the basis of the channelestimation result, the AP transmits the SDMA data 104 to the STA_1 110,the STA_2 120, and the STA_3 130 in SDMA transmission.

The STA_1 110, the STA_2 120, and the STA_3 130 receive the SDMA data104, and as an acknowledgement (ACK), transmit a block ACK 114, a blockACK 124, and a block ACK 134 to the AP.

In this case, an amount of data to be transmitted by the AP 100 to theSTA_1 110, the STA_2 120, and the STA_3 130 may be different among theSTAs. In other words, data frames each having a different length may besimultaneously transmitted to the STA_1 110, the STA_2 120, and theSTA_3 130, respectively. In this case, for effective utilization ofradio resources, multiple frame transmission may be achieved so that aplurality of data frames are consecutively transmitted within a range ofan SDMA transmission duration.

FIG. 2 shows an example of multiple frame transmission.

The SDMA data 104 of FIG. 1 may include SDMA data 210 to be transmittedto the STA_1 110 of FIG. 1, SDMA data 220, SDMA data 221, and SDMA data222 to be transmitted to the STA_2 120, and SDMA data 230 and SDMA data231 to be transmitted to the STA_3 130.

In the example of FIG. 2, the SDMA data frame 220 to be transmitted tothe STA_2 120 has a shorter length than an SDMA transmission duration.Accordingly, the SDMA data 221 and the SDMA data 222 can be furthertransmitted consecutively during the SDMA transmission duration.

In a case where the SDMA data 210 is transmitted to the STA_1 110 andthe SDMA data 220, the SDMA data 221, and the SDMA data 222 aretransmitted to the STA_2 120 during the SDMA transmission duration inmultiple frame transmission, whether synchronization is maintainedbetween different pieces of SDMA data simultaneously transmitted to therespective STAs may need to be considered.

FIG. 3 shows a problem in which synchronization is not maintainedbetween frames to be transmitted to respective STAs in SDMAtransmission.

FIG. 3 is an enlarged view of a portion 200 of FIG. 2. A data frameincludes an orthogonal frequency division multiplexing (OFDM) symbol(i.e., an inverse fast Fourier transform (IFFT)/fast Fourier transform(FFT) period of FIG. 3) and a guard interval (GI) for avoidinginter-symbol interference. According to the IEEE 802.11n standard, theIFFT/FFT period is 3.2 μs, and the GI is 0.8 μs. The IFFT/FFT period andthe GI will be collectively referred to hereinafter as an OFDM symbolduration. That is, the OFDM symbol duration is 4.0 μs in the IEEE802.11n standard.

In FIG. 3, transmission of SDMA data 210 to an STA_1 and transmission ofSDMA data 220 to an STA_2 start simultaneously. Until transmission ofthe SDMA data 220 ends, synchronization between the STA_1 and the STA_2is maintained since the OFDM symbol duration having the same length isrepeated. Although not shown in FIG. 3, synchronization is alsomaintained as to SDMA data 230 transmitted to an STA_3.

However, if the end of transmission of the SDMA data 220 is subsequentlyfollowed by the start of transmission of SDMA data 221, synchronizationbetween the SDMA data 210 and the SDMA data 221 is not maintained. Thisis because the SDMA data 221 transmitted subsequently to the SDMA data220 is transmitted when an RIFS 300 elapses after the transmission ofthe SDMA data 220. According to the IEEE 802.11n standard, the RIFS 300is 2 μs. Thereafter, synchronization between the SDMA data 210 and theSDMA data 221 is not maintained, which causes a problem in that aninterference level of a signal increases in a receiving STA side andimplementation complexity increases. Therefore, there is a need for amethod of maintaining synchronization between data frames transmitted torespective STAs when multiple frames are transmitted in a WLAN systemsupporting SDMA transmission.

FIG. 4 shows an example of a slotted RIFS proposed in the presentinvention.

It is described above that multiple frame transmission has a problem inthat synchronization is not maintained between frames transmitted torespective STAs. This problem occurs when an inter-frame space (IFS) isused as an RIFS in the multiple frame transmission. According to theIEEE 802.11n standard, the RIFS is 2 μs, which is not a multiple of anOFDM symbol duration.

To solve this problem, the present invention proposes to set the IFS toa multiple of the OFDM symbol duration. In this case, the IFS in themultiple frame transmission may be 0 μs, 4 μs, 8 μs, . . . , etc., whichis a multiple of the OFDM symbol duration (i.e., 4 μs), rather than theRFIS (i.e., 2 μs) based on the IEEE 802.11n standard. A new IFS set to amultiple of the OFDM symbol duration (i.e., 4 μs) proposed in thepresent invention will be hereinafter referred to as a slotted RIFS. Theterm ‘slotted RIFS’ is arbitrarily named.

A slotted RIFS 400 is set to 4 μs in FIG. 4 which shows multiple frametransmission according to an embodiment of the present invention. Unlikein the example of FIG. 3, the slotted RIFS 400 of 4 μs is used as an IFSbetween SDMA data 220 and SDMA data 221, and as a result,synchronization can be maintained between the SDMA data 210 and the SDMAdata 221.

The slotted RIFS proposed in the present invention can be used formultiple frame transmission in MU-MIMO. As to multiple frametransmission in SU-MIMO, an RIFS may also be used in addition to theslotted RIFS.

An AP may report to STAs whether the slotted RIFS is available. Forexample, a VHT operation information element including a slotted RIFSbit for reporting whether the slotted RIFS is available may betransmitted to the STAs. The VHT operation information element may betransmitted to the STA by being included in a probe response frame, abeacon frame, or the like based on the IEEE 802.11 standard. Uponreceiving the probe response frame or the beacon frame, the STA can knowwhether the slotted RIFS is available according to the slotted RIFS bitof the VHT operation information element. When the slotted RIFS bit isset to 0, the slotted RIFS is not available, and in this case, an IFSmay be set to an SIFS in multiple frame transmission. When conforming tothe IEEE 802.11a/n standard, the SIFS is 16 μs, which is a multiple ofan OFDM symbol duration (i.e., 4 μs). When the slotted RIFS bit is setto 1, the slotted RIFS is used as the IFS in the multiple frametransmission, and thus the multiple frame transmission occurs with aninterval of the slotted RIFS.

Meanwhile, in the IEEE 802.11n standard, a short GI is optionally usedto decrease an overhead. The short GI is 4 μs, and may be used in a datafield according to setting of a field for indicating whether the shortGI is available in a signal (SIG) field of a PLCP header. Since theshort GI is used in the data field, when using the short GI, an OFDMsymbol duration used in the PLCP header may differ from an OFDM symbolduration used in the data field. In other words, the OFDM symbolduration used in the PLCP header is 4 μs, whereas the OFDM symbolduration used in the data field is 3.6 μs.

In case of using the short GI, in order to maintain synchronization inmultiple frame transmission, the short GI has to be used in framestransmitted to all STAs. When the short GI is used in a frametransmitted to the STA_1 in the example of FIG. 1, the short GI also hasto be used in frames transmitted to the STA_2 and the STA_3. In otherwords, the same GI has to be used in all spatial streams in SDMAtransmission. In addition, since the short GI is used only in the datafield, multiple frame transmission is configured such that each framehas the same transmission start time. Therefore, when using the shortGI, the use of the slotted RIFS proposed in the present invention as theIFS is not a solution for the problem of not being able to maintainsynchronization in multiple frame transmission.

FIG. 5 shows an example of a synchronized multiple frame transmissionmethod proposed in the present invention.

In the synchronized multiple frame transmission method proposed in thepresent invention, transmission is achieved by synchronizing atransmission start time of each frame in multiple frame transmission. Inother words, when an AP transmits data frames 510 and 515 for multipleframe transmission to an STA_1 and transmits data frames 520 and 525 formultiple frame transmission to an STA_2, if transmission of the dataframe 520 ends first, the next frame 525 is not transmitted after anRIFS or an SIFS elapses. The AP waits until the transmission of the dataframe 510 ends, and starts to transmit the data frame 525 at the startof transmission of the data frame 515 to be transmitted after the dataframe 510. That is, a transmission start time of the data frame 515 anda transmission start time of the data frame 525 are aligned to a t_n+1570. In multiple frame transmission, transmission start times ofrespective frames are aligned to a t_n 560 and a t_n+1 570 as shown inFIG. 5. Transmission start times of frames to be transmitted to theSTA_1, the STA_2, and the STA_3 are aligned to each other, and even iftransmission for any one of the STAs ends first, a next frame is alignedagain to the transmission start time after transmission of the remainingSTAs ends. Accordingly, synchronization can be maintained between framesto be transmitted to respective STAs even if the short GI is used.

In the example of FIG. 5, to align transmission start times of the dataframes 515, 525, and 535 to the t_n+1 570, null data padding can beused. In order to allow the data frame 510 to have the same transmissionend time as the data frames 520 and 530 of which transmission endsbefore the data frame 510, a pad 521 is padded to the data frame 520,and a pad 531 is padded to the data frame 530. By using the null datapadding, lengths of the frames to be transmitted to the STA_1, theSTA_2, and the STA_3 can be adjusted to the same length.

As an example of the null data padding, the pad 521 and the pad 531 maybe a zero bit stream including no data. As another example, an aggregateMAC protocol data unit (A-MPDU) may be used as the null data padding.

FIG. 6 shows an example of an A-MPDU subframe format used in null datapadding according to an embodiment of the present invention.

An A-MPDU subframe used in the null data padding according to theembodiment of the present invention includes an MPDU delimiter 610, anMPDU 620, and a pad 630. Except when it is the last A-MPDU subframe inan A-MPDU, the pad field 630 is appended to make each A-MPDU subframe amultiple of 4 octets in length. The MPDU delimiter 610 may be 4 octetsin length. Table 1 shows an exemplary structure of the MPDU delimiter610.

TABLE 1 MPDU delimiter Size Field (610) (bits) Description Reserved(611) 3 Null (612) 1 Indicating that follow-up MPDU is null data. MPDUlength (613) 12 Length of the MPDU in octets CRC (614) 8 8-bit CRC ofthe preceding 16-bits. Delimiter 8 Pattern that may be used to detect anMPDU Signature (615) delimiter when scanning for a delimiter. The uniquepattern may be set to the value 0x4E.

The field names of Table 1 are arbitrarily named, and some of the fieldsmay be added or omitted. A null field 612 may have 1 bit in length, andwhen this bit is set to 1, it may indicate that a follow-up MPDU is nulldata.

The AP aggregates MPDUs to be transmitted to a specific STA. If there isno more MPDUs to be aggregated or if an A-MPDU size is no longer able tobe increased due to a limited A-MPDU size of a receiving STA, nullpadding is used to align a transmission start time. The AP sets the nullbit of the A-MPDU subframe to 1, and transmits null data for follow-upMPDUs.

The STA receives the A-MPDU, and evaluates a null bit of each A-MPDUsubframe of the A-MPDU. When the null bit is set to 1, it can be knowthat the follow-up A-MPDU subframe is null data, and thus the STA canimmediately discard the follow-up A-MPDU subframe without storing it ina buffer.

FIG. 7 is a block diagram showing an example of a wireless apparatus forimplementing an embodiment of the present invention. A wirelessapparatus 700 may be an AP or a non-AP STA.

The wireless apparatus 700 includes a processor 710, a memory 720, and atransceiver 730. The transceiver 730 transmits/receives a radio signal,and implements an IEEE 802.11 PHY layer. The transceiver 730 supportsMU-MIMO transmission by using multiple antennas. The processor 710 isoperationally coupled to the transceiver 730, and implements IEEE 802.11MAC and PHY layers. When the processor 710 processes an operation of anAP in the aforementioned method, the wireless apparatus 700 is the AP.When the processor 710 processes an operation of an STA in theaforementioned method, the wireless apparatus 700 is the STA.

The wireless apparatus' MAC layer implemented in the processor 710generates the aforementioned multiple frames, and generates an A-MPDU byaggregating the aforementioned A-MPDU subframes. The A-MPDU istransmitted to the transceiver 730 via a physical layer convergenceprotocol (PLCP) layer and a physical medium dependent (PMD) layer. TheMAC and PHY layers supporting the frame transmission method in multiplechannels of the present invention can be implemented by the processor710 and the transceiver 730 by modularizing each layer.

The processor 710 and/or the transceiver 730 may include anapplication-specific integrated circuit (ASIC), a separate chipset, alogic circuit, a data processing unit, and/or a radio frequency (RF)unit for mutually converting a baseband signal and a radio signal. Thememory 720 may include a read-only memory (ROM), a random access memory(RAM), a flash memory, a memory card, a storage medium, and/or otherequivalent storage devices. When the embodiment of the present inventionis implemented in software, the aforementioned methods can beimplemented with a module (i.e., process, function, etc.) for performingthe aforementioned functions. The module may be stored in the memory 720and may be performed by the processor 710. The memory 720 may be locatedinside or outside the processor 710, and may be coupled to the processor710 by using various well-known means.

The aforementioned embodiments include various exemplary aspects.Although all possible combinations for representing the various aspectscannot be described, it will be understood by those skilled in the artthat other combinations are also possible. Therefore, all replacements,modifications and changes should fall within the spirit and scope of theclaims of the present invention.

What is claimed is:
 1. A method of multi user multiple input multipleoutput (MIMO) communication in a wireless local area network (WLAN)system, the method being performed by a transmitting device andcomprising: generating, by the transmitting device, an Aggregate-MediumAccess Control Protocol Data Unit (A-MPDU) including an A-MPDU subframe,the A-MPDU subframe including a Medium Access Control Protocol Data Unit(MPDU) delimiter field; selecting, by the transmitting device, a type ofa guard interval to be applied to the A-MPDU among a short guardinterval and a long guard interval, wherein when the A-MPDU is generatedfor a plurality of receiving devices, the transmitting device selectsthe short guard interval only if all of the plurality of receivingdevices support the short guard interval; and transmitting, by thetransmitting device, the A-MPDU by using the selected guard interval,wherein the MPDU delimiter field includes a null field and an MPDUlength field, the null field indicates a corresponding A-MPDU subframeis used to pad the A-MPDU, and the MPDU length field indicates a lengthof an MPDU of the corresponding A-MPDU subframe, and wherein the nullfield is a 1-bit information field and a value of the null field is setto ‘1’ when the corresponding A-MPDU subframe is used to pad the A-MPDU.2. The method of claim 1, wherein the MPDU delimiter field is 4 byteinformation and is directly followed by the MPDU, which is directlyfollowed by a padding field.
 3. The method of claim 1, wherein the MPDUdelimiter field further includes a Cyclic Redundancy Check (CRC) field,which is directly followed by a delimiter signature field.
 4. The methodof claim 1, wherein each bit of the null data is set to ‘1’.
 5. Themethod of claim 1, wherein the transmitting device is an access point(AP) or a non-AP station.
 6. A transmitting device for multi usermultiple input multiple output (MIMO) communication in a wireless localarea network (WLAN) system, the transmitting device comprising: a radiofrequency unit configured to transmit the A-MPDU by using the selectedguard interval; and a processor coupled to the radio frequency unit andconfigured to: generate an Aggregate-Medium Access Control Protocol DataUnit (A-MPDU) including an A-MPDU subframe, the A-MPDU subframeincluding a Medium Access Control Protocol Data Unit (MPDU) delimiterfield; and select a type of a guard interval to be applied to the A-MPDUamong a short guard interval and a long guard interval, wherein when theA-MPDU is generated for a plurality of receiving devices, thetransmitting device selects the short guard interval only if all of theplurality of receiving devices support the short guard interval; andwherein the MPDU delimiter field includes a null field and an MPDUlength field, the null field indicates a corresponding A-MPDU subframeis used to pad the A-MPDU, and the MPDU length field indicates a lengthof an MPDU of the corresponding A-MPDU subframe, and wherein the nullfield is a 1-bit information field and a value of the null field is setto ‘1’ when the corresponding A-MPDU subframe is used to pad the A-MPDU.7. The transmitting device of claim 6, wherein the MPDU delimiter fieldis 4 byte information and is directly followed by the MPDU, which isdirectly followed by a padding field.
 8. The transmitting device ofclaim 6, wherein the MPDU delimiter field further includes a CyclicRedundancy Check (CRC) field, which is directly followed by a delimitersignature field.
 9. The transmitting device of claim 6, wherein each bitof the null data is set to ‘1’.
 10. The transmitting device of claim 6,wherein the transmitting device is an access point (AP) or a non-APstation.